The UN has accused armed groups in Syria of blocking the delivery of aid to the besieged city of Aleppo for "political gain" - including rebel factions inside Aleppo itself who say they will reject aid that comes through regime territory.

The UN has accused armed groups in Syria of blocking the delivery of aid to the besieged city of Aleppo for "political gain" - including rebel factions inside Aleppo itself who say they will reject aid that comes through regime territory.

Two days after the Syria ceasefire agreement went into effect, bombs have stopped falling on the rebel-held eastern half of Aleppo but the 275,000 people living there are still desperately short of food and medical supplies.

A convoy of 20 UN trucks carrying enough aid for 40,000 people is languishing at the Turkish border as diplomats try to secure agreement from both rebels and regime forces to allow the vehicles through.

"Some parties to the conflict are trying to use this for political gain," said David Swanson, a UN spokesman.

"The challenge for us is ensuring that all parties to the conflict are on the same page. If one element of the chain is not there we cannot proceed."

The UN would not say if the hold-up was being caused by the Assad regime or its rebel opponents, but at least part of the problem appeared to be inside the area of east Aleppo itself. Activists there said they intended to reject the UN aid in protest at the ceasefire agreement, which was brokered between the US and Russia, but without any input from the Syrian opposition.

Video from Aleppo showed a large demonstration against the UN had gathered at Castello Road, the key supply route that the aid convoy would have to travel down. At least some of the demonstrators were waving the black flags of Jabhat Fateh al-Sham (JFS), an al-Qa'ida-linked jihadist group formerly known as the al-Nusra Front.

Abdelkafe al-Hamdo, a teacher in Aleppo, said he disagreed with JFS but supported the protest against the UN aid because he felt the US-Russia deal allowed Bashar al-Assad, the Syrian regime leader, to keep a stranglehold on the city.

"This is a revolution of freedom, not a revolution of hunger," he said.

"We want to free ourselves from the slavery of Assad, not to eat when Assad allows the UN to feed us."

Residents also protested against the US agreement to jointly bomb JFS alongside the Russians if the ceasefire holds for seven days.

JFS are held in high regard by some residents in Aleppo after they successfully broke a regime siege in August.

Robert Ford, a former US ambassador to Syria and a champion of the moderate opposition, warned rebel groups that there was "no possible justification" for stopping the aid from getting into the city.

"The Syrian armed and political opposition will hugely damage their cause if they block aid," he said.